## Relation between Respiratory Quotient and Energy output

Human Nutrition 3(3+0)
Lesson 6 : Energy – Determination of energy requirements and energy sources

Relation between Respiratory Quotient and Energy output

The term respiratory quotient refers to the ratio between the volume of Co2 given out and the volume of O2 consumed by the human subjects.

When only carbohydrate is oxidized, the R.Q is 1.0 and when only fat is oxidized, the R.Q is 0.7 and when only protein is oxidized the R.Q. is 0.82. After an average meal containing 10 per cent protein, 20 per cent fat and 70 per cent carbohydrate, the R.Q. is about 0.82. The R.Q. is lowered to about 0.7 in diabetes mellitus as the body oxidizes large amounts of fat. Data regarding the quantities of carbohydrates and fats oxidized per liter of O2 with the corresponding non protein R.Q. and calories produced are given in table

Non-protein respiratory quotient, quantities of carbohydrates and fats oxidized and heat produced per liter of O2 consumed

(Data per liter of O2 consumed)

 Non-protein(RQ) Carbohydrate (g) Fat (g) Calories (kcal) 1.000.95 0.90 0.85 0.80 0.75 0.70 1.2321.010 0.793 0.580 0.375 0.173 0 00.091 0.180 0.267 0.350 0.433 0.502 5.0474.995 4.924 4.862 4.801 4.739 4.686

Calculation of amounts of proteins, fats and carbohydrates oxidized in the body
From the quantity of nitrogen excreted in urine and the quantity of CO2 produced and O2 consumed in 24 hours it is possible to calculate the amounts of proteins, fats and carbohydrates oxidized in the body. The following example will illustrate the principles involved in such a calculation:

Example: An adult excretes in 24 hours 8 g N (equivalent to 50 g proteins) in urine and uses 400 liters of O2 and gives out 320 liters of CO2. It is known that for 1 g of urinary N, 6.25g protein would oxidized and this will require 5.92 liters of O2 producing 38.0 liters of CO2. After deducting these from the total O2 consumed and CO2 produced, the O2 utilized and CO2 produced in the oxidation of fats and carbohydrates will be

400.0- 47.4 = 352.6 liters of O2 and

320.0-38.0 = 282.0 liters of CO2

It will be seen from table that at a R.Q of 0.80, 1 liter of O2 consumed would have oxidized 0.375 g carbohydrate and 0.350 g fat, 352.6 liters used would have oxidized 352.6x0.375g of Carbohydrate and 352.6x0.350g of fat.

Therefore the quantities of nutrients oxidized and energy produced in 24 hours are as follows:

 Energy (Kcal) Protein = 50gx4.1 205.0 Carbohydrates = 132.1gx4.1 543.3 Fat = 123.4gx9.3 1147.6 Total energy (Kcal) 1895.9